sbc_primitives: Fix build on non-x86 arch

Check if SBC_BUILD_WITH_MMX_SUPPORT is defined otherwise
sbc_init_primitives_mmx will not be declared and
_builtin_cpu_supports(“mmx”)) likely fail:

CC    sbc/sbc_primitives.lo
sbc/sbc_primitives.c: In function ‘sbc_init_primitives_x86’:
sbc/sbc_primitives.c:599:3: warning: implicit declaration of function
‘sbc_init_primitives_mmx’; did you mean ‘sbc_init_primitives_x86’?
 599 |  sbc_init_primitives_mmx(state);
   |  ^~~~~~~~~~~~~~~~~~~~~~~
   |  sbc_init_primitives_x86
sbc/sbc_primitives.c: In function ‘sbc_init_primitives’:
sbc/sbc_primitives.c:598:6: error: hwcap ‘mmx’ is an invalid argument to
builtin ‘__builtin_cpu_supports’
 598 | if (__builtin_cpu_supports(“mmx”))
   |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
make[1]: *** [Makefile:711: sbc/sbc_primitives.lo] Error 1
make: *** [Makefile:453: all] Error 2
2 files changed